1. Field of the Invention
The present invention relates to, in a mobile radio communication system in which a base station and a mobile station under control of the base station have a tracking channel for tracking each other by a narrow beam, a tracking method between the mobile station and the base station (namely, a visiting area base station as it controls an area which the mobile station moves around), and a method of searching an adjacent base station while the mobile station communicates with the visiting area base station.
2. Description of Related Art
FIG. 11 illustrates a configuration of a conventional “mobile radio communication system” described in Japanese Patent Application Laid-open No. Hei8-8814. In FIG. 11, reference numerals 1101a, 1101b and 1101c denote mobile stations; 1102 a base station; and 1103 an antenna of the base station 1102.
Next, the operation will be explained. In FIG. 11, different frequency channels are assigned to each mobile station. For example, an information channel f1, f2 and f3 are assigned to a mobile station 1101a, 1101b and 1101c, respectively. The mobile radio communication system further comprises a frequency channel f0 common to the base station for transmitting a beam direction information of antenna directivity. As shown in FIG. 12, the base station 1102 controls a transmitting direction omnidirectionally varying a bearing angle θ horizontally as θ1, θ2 and θ3. At the same time, as shown in FIG. 13, the base station 1102 controls a transmitting direction omnidirectionally varying a depression angle φ vertically as φ1, φ2 and φ3.
The mobile station 1101a or the like measures the above bearing angle and depression angle information as well as their receiving levels at any time. The mobile station 1101a or the like identifies a direction of a maximum measured receiving level as a direction of the base station, and transmits again an optimal bearing angle, an optimal depression angle, the receiving level of the optimal bearing angle and the receiving level of the optimal depression angle.
The base station 1102 determines a transmitting level based on the receiving level of the optimal bearing angle and the receiving level of the optimal depression angle transmitted from the above mobile station 1101a or the like, and controls the transmitting level to prevent interference among the base station 1102 and other base stations.
But, when the mobile station approaches to a boundary between the base station 1102 and an adjacent base station, there is a problem in that it is becoming difficult for the mobile station to select a beam from the base station because of a interference wave coming from a plurality of the adjacent base stations.
In “A radio communication system” described in Japanese Patent Application Laid-open No. Hei5-276084, a mobile radio communication system employing a directive antenna and a phased array antenna in a mobile station and a base station is shown, and further, two methods are described as a method of irradiating an omnidirectional pattern to start speech, one of the two methods is a method of providing a connection specific phased array antenna, and the other method is a method of adjusting a phase shifter of the array itself appropriately.
When one of the station performs a omnidirectional irradiation and the other station detects the irradiation in the above method, the other station performs a directive irradiation in the direction of the detected direction and then modifies a transmitting direction of a directive antenna at any time based on an information of a phase value and an amplitude value received by the phased array antenna. Further, a method is described that the above method enables a channel switching when the mobile station moves across the area of the other base station.
But, the above method always needs the omnidirectional irradiation, and needs to comprise both irradiation systems including the omnidirectional irradiation and the narrow beam irradiation.
As mentioned above, in conventional mobile radio communication system, when the mobile station approaches to a boundary between the base station and an adjacent base station, there is a problem in that it is becoming difficult for the mobile station to select a beam from the base station because of a interference wave coming from a plurality of the adjacent base stations.
In another conventional mobile radio communication system, there is a problem in that both of the omnidirectional channel and the narrow beam channel are needed to start a speech.
Further, in an overlapped area in which a mobile station can communicates with a plurality of base stations, both of an adjacent base station which may be handovered and a mobile station need a tracking channel different from the above channel. In the overlapped area, there is a problem in that the adjacent base station and the mobile station set the tracking channels of different frequency and different beam to avoid the interference and track each other, resulting in increasing tracking time.